1. Field of the Invention
The invention relates to apparatus for suppression of rain echoes in radar equipment.
2. Description of the Prior Art
When, in radar equipment, a point target to be detected and rain echo signals are found in the same radar resolution cell, considerable interference is caused. This rain interference becomes stronger as the radar wavelength used becomes shorter.
To overcome this problem, it has been common practice in prior art radars to employ circular polarization. In such arrangements, the radar transmits a circularly polarized wave obtained by means of a polarizer, such as the quarter-wave plate type, placed in front of the aperture of a linearly polarized horn antenna, for example with vertical polarization. The quarter-wave plate is orientated at .pi./4 to the vertical. The radiated electrical field component which is perpendicular to the quarter-wave plate is not affected, whereas the component which is parallel to the said plate is delayed in phase by .pi./2. Consequently, the output wave from the polarizer will be circularly polarized. Assuming that the rain is a perfectly isotropic medium in polarization and that the radar antenna is perfect, the wave reflected from the rain will still be circularly polarized. After passing through the polarizer, the component of the electrical field which is parallel to the quarter-wave plate will again be delayed in phase by .pi./2 and, after recombination with the component which is perpendicular to the said plate, will produce a horizontally polarized field which will not be accepted by the vertically polarized horn waveguide feeder. The rain echoes are thus eliminated while the target echo, in general anisotropic in polarization, is no longer circularly polarized and gives a useful signal not cancelled out.
In practice, the antenna is not perfect and the transmitted polarization is not exactly circular. Normally it is at least somewhat elliptical and characterized by an ellipticity radio .tau. defined as the ratio of the major axis to the minor axis of the ellipse defined by the electrical field vector. Consequently, the rain echoes are not completely eliminated. Thus, for example, for .tau. = 0.9 dB (typical value), the suppression factor is 20 dB.
It has been assumed up to this point that the rain was a medium perfectly isotropic in polarization but actually it has at least some depolarizing effect. Measurements and calculations have shown that the rain is an anisotropic medium whose principal axes are the vertical axis and the horizontal axis. The horizontal component of the electrical field is attenuated more and delayed more than the vertical component. Consequently a circularly polarized transmitted wave will be changed into an elliptically polarized wave after crossing through a rain zone. The ellipticity ratio increases with the thickness of the rain zone traversed and the intensity of the precipitation. Thus, for example, for a rainfall having an intensity of 12.4 mm/h present over a 10 km path, the attenuation of the rain echoes is limited to 10.4 dB in the X band and 15.9 dB in the C band.
The manner in which the present invention deals with this problem by providing novel apparatus and function will be understood as this description proceeds.